Why I can't select the vertices of both my objects?
Blender only allows us to edit one object at a time (2.80 will add the ability to edit multiple objects). By having your mesh in different objects you need to edit them separately.

One option is to join the objects and edit them as one &#9096; CtrlJ. After you have finished you can separate them again if you wish with P and choosing by selection or loose parts.

Another option is to use snapping to align the verts of one object with the verts of another.